THE SEEDLING AND YOUNG PLANT.
numerous transverse sections at different levels of the root would prove that the smallest vessels are the first to develop; whence we learn two facts—namely, that the xylem vessels of the young root are developed in centripetal order, and that the later ones have a larger caliber than those formed earlier.
If longitudinal sections are compared with these transverse ones—and I may here observe that it is only by means of numerous such comparisons that these matters have been gradually discovered—it is found that each vessel is a long tube, usually containing air and water when complete, the lateral walls of which are curiously and beautifully marked with characteristic thick and thin ornamentation. It must suffice here to say that the small, outer, first-formed vessels are marked with a spiral thickening, reminding one of caoutchouc gas-tubing kept open by means of a spiral wire inside; while the larger ones, developed later, usually have numerous small pits on their walls, reminding one of mouths, and the structure of which is very curious. Consequently these groups of xylem vessels are said to consist of spiral and pitted vessels, and their chief function is to convey water up the root to the stem (cf. Fig. 16). Packed in between these vessels are certain cells known as the wood-cells.Returning to the transverse section, we saw that between each xylem group described above there is a group of structures differing from the latter in their less distinct outlines; these alternate groups are known as